Pulse radar altimeters demonstrate superior altitude accuracy due to their inherent leading edge return signal tracking capability. The pulse radar altimeter transmits a pulse of radio frequency (RF) energy, and a return echo is received and tracked using a closed-loop tracking system. Mission requirements for covertness and higher sensitivity, along with technological advances in terms of microminiaturization and the cost of special purpose integrated circuits have resulted in the feasibility of the more complex coherent pulse Doppler radar.
Coherent pulse Doppler radars normally incorporate a digital code which is used to biphase modulate the transmit pulse. Systems of the prior art have required relatively long code lengths and long pulse repetition intervals to provide the ambiguous range rejection required for a high range radar. The long code length, however, requires a receiver with a narrow bandwidth integration filter, resulting in insufficient high Doppler frequency processing capability required to track ground returns under high climb or dive rates and sudden terrain changes.
Prior art coherent pulse Doppler radar altimeters exhibit attributes of superior covertness, processing gain, and leading edge tracking accuracy, but suffer from insufficient sensitivity at the higher altitude climb and dive rates. This results in an altimeter which will not track the ground return under sudden terrain altitude changes, or high dive or climb conditions. Additionally, non-coherent pulse radar altimeters of the prior art provide less than a 1% duty cycle. The present invention exhibits increased sensitivity with greater than 30% duty cycle while transmitting less peak power. Thus, 140 dB loop sensitivity is achieved with a 50 milliwatt transmitter compared to presently achieved sensitivities of less than 140 dB with a 5 watt transmitter. This ability to operate at greatly reduced power, of course, enhances covertness.
The interval of time between signal bursts of a radar system is called the pulse repetition interval (PRI). The frequency of bursts is called the pulse repetition frequency (PRF) and is the reciprocal of PRI. Prior art systems that utilize a short PRI in order to be able to process high Doppler rates suffer from an ambiguous range problem.
FIG. 1 graphically demonstrates the ambiguous range problem associated with a short pulse repetition interval system. Illustrated in the line labeled XMIT FORMAT are three transmitted pulses labeled T.sub.A, T.sub.B and T.sub.C which could be used with a 10,000 foot range capability radar altimeter. A 15,000 foot pulse repetition interval is used to provide up to 2,000 feet/second altitude rate change capability. If the aircraft is flying at 6,000 feet, a return R.sub.A will appear delayed 6,000 feet after each transmission and the altimeter will indicate a true 6,000 foot altitude. As further illustrated in FIG. 1, a 21,000 foot delay, when flying at 21,000 feet will also appear 6,000 feet after the second transmission T.sub.B, resulting in 6,000 feet being indicated when the aircraft is actually at 21,000 foot altitude. Thus, an ambiguous range exists at 21,000 feet and, in a like manner, at 36,000 feet which is 6,000 feet following the transmitted pulse T.sub.C.
FIG. 2 serves to demonstrate the ambiguous range problem associated with a short (13 bit) code or pulse length. Improper line-up or correlation of the demodulation code with the biphase coded ground return will result in an altitude error as indicated. This error can be as high as approximately 1,000 feet for a two microsecond (1,000 radar feet) wide pulse. Proper line-up or correlation of the demodulation code with the ground return will result in the correct altitude being reported. The degree to which the radar receiver processing system rejects improper code line-up or correlation is called "auto-correlation rejection".
The level of rejection is directly dependent upon the number of bits in the code, and the type of code employed. A 13-bit Barker code, for example, provides about 22 dB auto-correlation rejection, while a 5-bit Barker code provides about 14 dB rejection. Because the strength of the signal return from the ground can vary as much as 50 dB due to terrain reflectivity variances and aircraft roll, these short code lengths would result in a system incapable of rejecting the ambiguous range associated with miscorrelation of the coded ground return with the receiver demodulation code. Thus, a radar altimeter designed to process high climb, dive and terrain rate changes must necessarily have a short PRI and a short code, resulting in ambiguous range reporting due to the two effects discussed above.